One of the major functions of double-apron drawing mechanisms of textile machines is creating a space for a fibrous material being treated, wherein there are produced friction forces both between the fibres of the fibrous material itself and between the fibres of the material and the elements of the drawing mechanism. These friction forces should be sufficient to shape the relatively dense mass of the treated fibrous material. The space wherein the friction forces are produced in the abovedescribed manner is sometimes called the friction force field having a certain intensity value. The intensity value of such a friction force field is the mean friction force applied in the respective area of the friction force field to one millimeter of the treated fibre in the direction of the travel of the fibrous material. This intensity value varies in dependence on the value of the load acting perpendicularly to the travel of the treated fibrous material, and also on the thickness and the structure of this fibrous material.
There is known a drawing mechanism (cf. the SU Inventor's Certificate No. 364,696) wherein the treated fibrous material is made to pass between two endless aprons or belts of the feed couple, synchronously rotated about rolls or rollers, wherefrom it is directed into the delivery couple.
One endless apron is associated with a stationary guide extending longitudinally of the travel of the fibre, while the other endless apron is associated with a movable guide arranged adjacent to the area of exit of the fibrous material from the feed couple toward the delivery couple. The movable guide provides for stepwise or incremental adjustment of the gap between the endless aprons in the area of exit of the treated fibrous material from the feed couple in a vertical plane extending transversely of the travel of the treated fibrous material.
It is understandable that the adjustment of the gap between the aprons is essential for creating the optimum distribution of the friction force files in cases of fibrous materials differing by their composition, structure and thickness, to enhance the process of drawing the fibrous material, as the latter is being transferred from the feed couple into the delivery one.
This, in its turn, is essential for improving the yarn quality. And from this point of view it should be stated that the aforementioned drawing mechanism of the prior art is not free from drawbacks.
Firstly, in this mechanism the movable guide provides for but stepwise or incremental adjustment of the gap between the endless aprons in the area of exit of the fibrous material being treated from the feed couple. This limitation would not enable to ensure the optimum distribution of the friction force fields for the entire broad range of the thicknesses of the treated fibrous material and of its possible compositions, and hence would eventually affect the drawing process and cause unevenness of the yarn.
Secondly, the adjustment of the gap between the endless aprons in the area of exit of the treated fibrous material from the feed couple into the delivery one is effected in the drawing mechanism of the prior art solely by the angular adjustment of the movable guide in a vertical plane extending longitudinally of the travel of the fibrous material being treated.
Thus, in case the fibrous material treated in the double-apron drawing mechanism is droplet-shaped in cross-section, i.e. one end portion of this treated fibrous material in cross-section is substantially thicker than the other one, the thinner end or side might all but fail to engage one of the endless aprons. Consequently, there cannot be ensured the optimum distribution of the friction force fields in the body of the material being treated throughout the width thereof. This eventually results in that the material being drawn from the feed couple is made up of not individual or discrete fibres, but of groups of fibres, which also affects the quality of the year to be produced.